1. Field of the Invention
This invention relates to a control for energizing a heater provided to raise the temperature of the lubricating oil in a compressor, and, in particular to a system wherein the heater comprises the coils of an electric motor and the coils are selectively energized to a non-rotating condition in response to sensed temperatures.
2. Description of the Prior Art
It is well known that under certain conditions, some refrigerants and the oil used as a lubricant for the compressor of a refrigeration unit are freely miscible. During normal operation of the refrigeration circuit, because of operating pressures and temperatures, the oil in the sump of the compressor, will be substantially free of refrigerant. However, when the compressor is shut off, refrigerant in the system tends to migrate and condense in the coldest parts of the system. As an example, when a system is shut down on a cool night, the temperature will be warmer inside and the refrigerant in the system will tend to migrate to the cool outdoor coil and the compressor. In the morning as the outdoor temperature goes up, the outdoor coil will warm up far quicker than the compressor because of the compressors large thermal mass. As a result refrigerant which had condensed in the outdoor coil will migrate to the compressor. Under such conditions there is a danger that the oil in the sump will be diluted and the risk is high for a flooded compressor start due to the presence of liquid refrigerant in the compressor sump. Crankcase heaters have been designed to keep refrigerant from migrating into the crankcase or sump of a hermetic compressor during an off cycle. Some crankcase heaters are normally energized continuously even though they are only effective part of the time. Such heaters may be electrical resistance elements which are installed directly in the sump of the compressor, or may be wrapped around the outer surface of the compressor casing in heat transfer relation with the oil stored in the sump.
It has been found that heating elements for this type of crankcase heater are expensive. Since hermetic compressors are standard equipment in many refrigeration systems today, and since the motor and crankcase are in a common space in these compressors, use of the motor windings has been suggested for use as a heating element to heat the oil within the compressor. U.S. Pat. No. 3,133,429 "Compressor Crank Case Heating Device" discloses such a crankcase heating system.
U.S. Pat. No. 4,066,869 "Compressor Lubricating Oil Heater Control" discloses a thermostatically operated switch associated with a crankcase heater for selectively connecting the heater to a source of electrical energy. The switch is responsive to a temperature indicative of lubricating oil temperature and to operation of the compressor. The switch energizes the heater when the sensed temperature falls below a predetermined level and the compressor is not in operation. The switch de-energizes the heater when the compressor is operable regardless of the temperature of the ambient. U.S. Pat. No. 5,012,652 "Crank Case Heater Control for Hermetic Refrigerant Compressors" discloses another crankcase heater control to selectively energize the crankcase heater when needed to therefore reduce the energy consumed by the heater. The '652 patent senses temperature at the compressor, the indoor coil, and ambient temperature. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures the crankcase heater is de-energized.
U.S. Pat. No. 4,638,643 "Control System for Compressor Motor Used With Air Conditioning Unit" discloses a control system that uses compressor motor windings to preheat a compressor on actuation of a operation switch.